Soil DNA extraction
DNA was extracted from 5 g of soil samples stored at -80 °C, following the instruction of PowerMax Soil DNA Isolation Kit (MoBio Laboratories, Carlsbad, CA, USA). DNA quality was assessed via a NanoDrop ND-1000 Spectrophotometer (NanoDrop Technologies Inc., Wilmington, DE, USA), based on UV spectrometry absorbance ratios at wavelengths of 260 nm/280 nm and 260 nm/230 nm. DNA concentration was measured by PicoGreen using a FLUOstar OPTIMA fluorescence plate reader (BMG Labtech, Jena, Germany).